1. Technical Field
The present invention relates to speech scramblers.
A speech scrambler is used to scramble an input speech signal so that it can be securely sent along a telephone line. It is necessary that the signal sent along the telephone line be an analogue signal of the same bandwidth as a normal speech signal but that it be unintelligible to anybody tapping onto the line. However, the speech should be intelligible to the other party to the conversation who is equipped with a compatible de-scrambler.
2. Prior Art
Various types of speech scrambling systems are known. For example, many systems use digital processing by sampling the input speech signal at fixed time intervals to produce a block of time samples. In one system the blocks of time samples are simply rearranged and converted back into an analogue signal for transmission. Such a system is a time domain scrambler. This system requires synchronisation between the transmitting scrambler and the receiving de-scrambler in order to achieve an acceptable speech quality at the output of the de-scrambler.
In a typical frequency-domain scrambler, the block of time samples is converted by a fast Fourier transform to produce a series of Fourier coefficients representing the frequency spectrum of the input speech signal. If these Fourier coefficients are permuted before being subjected to an inverse fast Fourier transform, a new block of time samples is produced which can be converted into a scrambled analogue signal for transmission. At the receiver the input signal is again sampled and these samples subjected to a fast Fourier transform. The resulting coefficients are permuted in the inverse manner to the permutation applied by the scrambler and subjected to a Fourier transform and this produces a sequence of time samples which should convert to the original input speech signal.
Such systems have certain disadvantages. In particular the output signal may well contain unwanted high frequency components. These are produced as a result of the block processing of the input speech signal which means that there may be waveform discontinuities at the "joins" between the blocks output by the scrambler. Various windowing techniques have been employed to attempt to overcome these defects.
Such a basic frequency-domain scrambler also requires synchronisation between the scrambling systems employed at each end of the link. This has been overcome in recent designs by the use of short time Fourier transforms (STFT). Such a scrambling system is described by L. S. Lee et al in a paper entitled "A new frequency-domain speech scrambling system which does not require frame synchronisation" in IEEE Trans., Com-32, No. 4, April 1984.
The latter system may basically be regarded as continuously passing the input signal through a bank of frequency shifters, the output of each frequency shifter being passed to an ideal low pass filter. The outputs of the filters are then permuted. The scrambled frequency spectrum is then reconverted into an analogue signal for transmission.
The requirement in such a system to carry out the necessary windowing of both the input signal and the scrambled output, together with the intermediate STFT processing of the samples, imposes a considerable requirement for processing power. This makes such systems very expensive.
Another system which operates on time-domain samples is described in a paper entitled "An efficient time-domain sample value scrambling scheme eliminating frame synchronisation requirement for secure speech communications" by G. L. Chou and L. S. Lee in Global Telecommunications Conference, December 1982. This system does not require synchronisation, but still requires both the input and scrambled output to be windowed besides the intermediate permutation operation and, therefore, still requires considerable processing power.